This invention relates to a novel cathode-ray tube having a viewing screen and means for defining to a viewer an array of discrete excited areas of the screen.
Color television picture tubes which include means for defining an array of discrete excited areas of the viewing screen have been described previously. In one form, referred to as a positive-tolerance tube, the defining means includes an apertured mask located between the cathode and the screen. The apertures of the mask shadow the screen and thereby define the excited areas of the screen. In another form, referred to as a negative-tolerance tube, the tube includes an apertured mask located between the cathode and the screen and also a light-absorbing matrix adjacent the screen and between the screen and the viewer. The matrix has an array of holes therethrough which define to the viewer excited portions of the screen.
At first, the positive-tolerance tube comprised a round envelope, a round screen and a round mask with apertures of substantially uniform size. Later, it was proposed to grade the size of the apertures, and therefore the excited areas of the screen, with the smallest apertures at the peripheral portions of the mask and the largest apertures at the central portion of the mask. This is sometimes referred to as radial or circular grading. Apertures of the same size were located in a circle at the same distance from the screen center. With increased aperture size at the central portion of the screen, the tube exhibits greater overall brightness to the viewer. With smaller aperture size at the peripheral portions of the screen, the tube exhibits greater tolerance to misregister. See, for example, U.S. Pat. Nos. 2,755,402 to A. M. Morrell and 3,109,116 to D. W. Epstein et al.
Picture tubes comprised of rectangular envelopes, rectangular screens and rectangular masks have replaced substantially all of the round tubes. However, where grading of the aperture sizes in the mask is used, circular grading is still used. This follows the general belief that optimum performance is achieved with circular or radial symmetry about the longitudinal axis of the tube. With the introduction of negative-tolerance tubes, similar circular grading was used for the light-absorbing matrix for essentially the same reasons. Nevertheless, further improvements in brightness and/or misregister tolerance are desirable for both positive- and negative-tolerance tubes.